vaapivideobufferpool: always update/release the underlying surface proxy
gst_vaapi_video_buffer_pool_reset_buffer() is called when the sink releases the last reference on an exported DMA buffer. This should release the underlying surface proxy. To avoid releasing the wrong surface due to a stale surface proxy reference in the buffer's GstVaapiVideoMeta, always update the reference to the correct surface in gst_vaapi_video_buffer_pool_acquire_buffer().